The present invention relates to a color cathode ray tube, and particularly to a shadow mask with its shape of a curved surface in an apertures region thereof.
The legibility of a picture can be improved by making the panel outside surface flat. If the panel outside surface is made flat, the panel inside surface becomes close to a flat surface. Since the shadow mask should be approximately parallel with the curved panel inside surface, the shadow mask also becomes approximately flat. Since the shadow mask is formed by pressing, it is stronger when it has larger curvature (i.e., it is weaker when it is closer to a flat member). Therefore, in color picture tubes having a flat outside surface, insufficient strength of the shadow mask is a serious problem.
U.S. Pat. No. 4,136,300 (Morrell) discloses a technique of suppressing doming by making the curvature of the shadow mask larger that that of the panel inside surface by changing the shadow mask pitch in the screen peripheral portion from that at the screen center.
JP-A-51-47365 discloses that, to closely arrange phosphor dots on the panel inside surface, the panel inside surface or the shadow mask is formed by a plurality of concentric spherical surfaces or the radius of curvature of the panel inside surface or the shadow mask is varied clockwise in a plane that is perpendicular to the tube axis.
U.S. Pat. No. 4,839,556 (Regland) discloses a technique of suppressing doming by making the curvature in the screen peripheral portion larger than that at the screen center on the major axis of the shadow mask.
However, the U.S. Pat. No. 4,136,300 does not disclose how to change the shadow mask pitch in the screen peripheral portion from that at the screen center. If the shadow mask pitch in the screen peripheral portion were made too larger than that at the screen center, the resolution would decrease unduly in the screen peripheral portion.
Further, neither of the above techniques cannot provide sufficient mask strength for color cathode ray tubes whose panel has a flat outside surface.
According to a first aspect of the invention, in a color cathode ray tube whose panel has a flat outside surface, the radius of curvature of the shadow mask curved surface decreases approximately linearly as the position moves from the center to the periphery.
Decreasing the radius of curvature approximately linearly can increase the strength of the shadow mask.
According to a second aspect of the invention, in a color cathode ray tube whose panel has a flat outside surface, the radius of curvature of the shadow mask curved surface decreases as the position moves from the center to the periphery and radii of curvature on the screen major axis, minor axis, and diagonal axis at positions on a circle centered by the shadow mask center and having an arbitrary radius do not have large differences.
This can also increase the strength of the shadow mask.